1. Field of the Invention
The present invention relates generally to annulus pressure responsive downhole tools utilizing a compressible liquid spring.
2. Description of the Prior Art
The prior art includes a number of downhole tools, such as flow tester valves and circulating valves, which are designed to operate in response to changes in pressure in a well annulus between a tool string and a well casing. Typically, these tools include a differential area piston, which may generally be referred to as a power piston, having one side communicated with well annulus pressure and having another side communicated with a compressible fluid spring chamber.
The compressible fluid spring chamber typically has been filled either with a compressible gas such as nitrogen or a compressible liquid such as silicone oil.
When well annulus pressure is increased to move the power piston of the tool, the fluid in the spring chamber is compressed. Upon decreasing the well annulus pressure, the compressed fluid in the spring chamber expands to aid in returning the power piston to its original position.
Typical examples of prior art tools utilizing compressible nitrogen spring chambers are seen in U.S. Pat. Nos. 4,422,506; 4,429,748; 4,489,786; and 4,515,219, all to Beck and all assigned to the assignee of the present invention.
These prior art tools utilizing compressible nitrogen gas spring chambers rely substantially entirely upon the compression of the nitrogen gas to accommodate the displacement of the power piston of the tool. Although these nitrogen gas tools may have a volume of oil in fluid pressure communication with the nitrogen gas for the purposes of metering the oil through a metering cartridge to provide a time delay in transmission of changes in well annulus pressure to the nitrogen, this oil does not provide any substantial volume change upon movement of the power piston. This is true even when silicone oil is used in these tools, because the volume of oil subject to compression upon movement of the power piston is insignificant as compared to the very large volume of pressurized nitrogen gas which is provided.
One significant problem with tools relying upon compression of nitrogen gas is that the nitrogen gas must be initially placed in the tool at a relatively high pressure on the order of, for example, 4,000 psi. In typical tools such as those shown in U.S. Pat. No. 4,422,506 to Beck, when utilized at hydrostatic well annulus pressures in a range of 2000 to 8000 psi, the initial nitrogen charge pressure will correspondingly range from 1255 psi to 4476 psi. There are dangers of explosion inherent in the assembly and charging of such tools, as are present in any high pressure vessel. These problems are exacerbated by the fact that sometimes nitrogen supplies are not sufficiently pure and contain excessive amounts of oxygen which may cause detonation when these high initial charge pressures are applied to the tool. Additionally, if such an explosion does occur, the large volume of highly pressurized gas provides a very large volume expansion during the explosion which propels pieces of the disintegrating tool with high energy and thus creates a significant danger.
Two prior art circulating valves utilizing compressible silicone oil spring chambers are shown in U.S. Pat. Nos. 4,109,724 to Barrington and 4,109,725 to Williamson et al., both assigned to the assignee of the present invention.
Two prior art tester valves utilizing silicone spring chambers are shown in U.S. Pat. Nos. 4,444,268 and 4,448,254, both to Barrington and both assigned to the assignee of the present invention.
These prior art tools which utilize compressible silicone oil to accommodate the displacement of the power piston require a very large volume of silicone oil due to the relatively low compressibility of silicone oil. This means that the tool must be relatively large. Even though such tools when assembled may have a relatively small mass of air trapped in the tool which becomes compressed along with the silicone oil, the mass of air present in the tool is insignificant as compared to the mass of oil present in the tool, and thus the trapped air does not significantly contribute to the volume change which must be provided to accommodate the displacement of the power piston of the tool.